Design

Statutory requirements

The following standards deal with drainage design:

• BS EN 752: Parts 1-7:1997 Drain and sewer systems outside buildings.
• BS EN 1610: 1998 Construction and testing of drains and Sewers
• BS EN 12056:2000 Gravity drainage systems inside buildings, parts 2 & 3

Marley underground drainage products may also be used in drainage systems which are designed in accordance with BS8301 (the previous standard).

The design and layout of drainage and sewerage systems should comply with The Building Regulations and Water Authority Specification.

Reference should also be made to the Sewers for Adoption manual

Design methods

The following documents detail calculation methods for the structural design of underground flexible pipelines:

• BS EN 1295-1:1998, Structural design of buried pipelines under various conditions of loading.
• 'The materials selection manual for sewers' published by the Foundation for Water Research and Water UK.

These guides recommend initial and long-term stiffness values for BS EN 1401: 1998 PVCu pipes, which should be adopted when performing calculations for the Marley solid wall drainage products. Quantum structured wall pipe has a higher stiffness value which should be substituted when carrying out calculations relating to the use of Quantum Sewer.

Flexible pipe theory

PVCu pipes derive their load bearing capacity from a combination of their inherent stiffness and the characteristics of the bedding and sidefill material. When a vertical load is imposed on a flexible pipeline, it will deform and the resulting horizontal force is transmitted to the undisturbed trench wall via the sidefill material.

The design of a PVCu pipeline must therefore take into account the interaction between the pipe and bedding material. This will enable PVCu pipes to withstand imposed loads without excessive deflection whilst also, unlike rigid pipes, maintaining their ability to cope with possible ground movement without fracturing.

Deformation

To ensure satisfactory performance of a PVCu pipeline, it is generally recommended that a limiting negative deformation of 6% of the vertical diameter of the pipe is used in calculations. This level of deformation has very little effect on the flow carrying properties of the pipeline. A 6% deformation will cause less than a 1% reduction in full bore capacity. Practical experience and test work indicate that the vast majority of any deformation will occur within the first two years following installation and any increase thereafter will be negligible.